Ellipsometry is a well-known measurement technique for measuring polarization changes in a light beam caused by interaction with an article (i.e., reflection from or transmission through the article). Due to high accuracy of ellipsometric measurements and due to high sensitivity of ellipsometric parameters to thin films (articles), these measurements are widely used in different fields, including biological and medical applications, as well as electro-chemistry and microelectronics. Ellipsometry has been used in the semiconductor industry to measure thickness of very thin films, such as silicon dioxide films, silicon nitride and the like, and thus to control critical processes of semiconductor manufacturing like thermal oxidation, chemical-vapor deposition, etc.
Spectrophotometric measurements (also termed reflectometry, spectrometry, scatterometry) are also widely used techniques for measuring thickness and optical properties of thin films and patterns in semiconductor manufacturing. Usually, spectrophotometry is applied for measuring of thicker films than ellipsometry, but spectrophotometry has substantial advantages in many aspects like a spot size, throughput, cost, etc.
Different types of spectrophotometers and ellipsometers are known in the art and disclosed for example in “Ellipsometry and Polarized Light”, R. M. Azzam et al., 1986. Ellipsometric and spectrophotometer systems typically include a stationary optical head and an appropriate support assembly, for example having two-coordinate stage for holding and locating an article under measurement relative to a light spot.
The reflectometry and ellipsometry systems can be implemented to operate in both normal and oblique incidence of a light beam onto the article plane. The incidence plane (IP) is defined in space as a plane perpendicular to the article plane and including an incident light beam and a returned (e.g., reflected) beam. In case of optically isotropic articles, measurements are independent of the IP orientation relative to the article. In case of optically anisotropic articles, e.g. gratings, relative orientation of the IP and the article's features should be well defined in order to ensure correct measurement conditions.
Semiconductor wafers are typically in the form of an array of identical patterned regions (termed “die”) arranged on a substrate. In case of a semiconductor wafer, if a measurement site within or near a die is a grating (line array) oriented along X- or Y-axis and a wafer is oriented on an X-Y-stage at a required angle relative to the IP, all such sites along the wafer surface are oriented at the same angle so measuring different sites by moving the stage from one location to another is carried out under the same conditions.
Recently, polar stages for wafer movement have been used in some optical systems mainly for reaching a small system footprint, allowing using them in integrated metrology (i.e., installing them into a limited space of processing equipment). In order to measure different sites on a wafer, the polar stage should move the wafer by both translation (R-coordinate) and rotation (Theta-coordinate). In this case, each measurement site will be oriented differently relative to the IP of the optical head of an oblique angle reflectometer or ellipsometer, thus not allowing the same measurement condition for anisotropic sites. For this reason, polar stages are typically used only with normal incidence measurement devices, while oblique incidence devices like ellipsometers utilize only X-Y-stages.
U.S. Pat. No. 6,859,279 discloses a method of measuring a small area on a substrate with an ellipsometer. According to this technique, a substrate is oriented with respect to the ellipsometer such that an elliptical light spot produced by the ellipsometer fits diagonally within the test area. Then, the surface properties of the substrate within the test area are measured with the ellipsometer.